Japan’s move to ease restrictions on American beef imports will allow ranchers and meatpackers to move past a mad cow scare that led to a ban in 2003, and regain full access to what was once their most lucrative market.

The Department of Agriculture announced that it had identified a case of mad cow disease, the first in six years, in a dairy cow in central California. The cow ''was never presented for human consumption, so it at no time presented a risk to the...